Because he feels like HE did something wrong or that he behaved in a manner that gave you the NOTION he would be interested in you. It makes him uncomfortable knowing that you crush on him. If his intentions was to be friendly with his "student" and you construed it into something more it makes people uncomfortable.

Having a crush is fine, it's normal - BUT that doesn't MEAN you HAVE to tell the person you are crushing on. SPECIALLY not when you know NOTHING will come of it, because? IT MAKES the other person absolutely uncomfortable.

What WAS the point of telling him? IT was for YOUR benefit, not his. THAT is why thinks are weird now.